fix(fuse): cache an upload whose read-back failed, and know it as ours (B157)

When the read-back after an upload went unanswered, the drain let go of
the staged blob without caching it. A partial write opened next had
nothing to fill its gaps from and queued an incomplete revision, so the
write open after it was refused with EIO.

The bytes are now cached under the size and mtime the tree still has,
which is the base the next write takes. Their SHA-1 stands in for the
revision id the read-back did not bring, so that write chains onto the
upload and does not fork a conflict copy of itself.

docs/MILESTONE-3.0.0-STATUS.md

@@ -9,7 +9,7 @@ Tracks `docs/MILESTONE-3.0.0.md`. Update it with every commit that moves a phase
 - **Phase 2**: local ids, inode = local id, one tree for every mount (2.10.0).
 - **Phase 3, first part**: mkdir, create, rename, unlink and rmdir on the mount are recorded
   locally and sent from the queue; `"local_first": false` brings back the old path (5e0f30c).
-- Fixes from the simulation and account runs since then: B127 and B141 to B156.
+- Fixes from the simulation and account runs since then: B127 and B141 to B157.
 - **Sync issues, first cut** (§6.2): refusals sorted into issues, shown at once in the app,
   tray and CLI; Export button and `pdfs sync export`; `pdfs sync issues`.
 
@@ -79,7 +79,9 @@ Tracks `docs/MILESTONE-3.0.0.md`. Update it with every commit that moves a phase
          a write whose answer was lost (B155). Both fixed. One slow-link run of seed 2 failed
          once with a rename answering `ENOENT` as a folder move landed. Not reproduced since;
          it may be B152. Replays of slow-link seed 4 failed two of six times on a file written
-         again after its create lost its answer (B156, fixed).
+         again after its create lost its answer (B156, fixed). With sixteen drain workers
+         (16fca10), seed 4 failed every replay on a write after an upload that was not read
+         back (B157, fixed).
    - [ ] Known-bug list in `sim/run.rs` is empty: B125 and B130 open; B127, B129 and B132 fixed
          but unconfirmed.
    - [ ] A clean account run. The one on a6f36d5 (2026-10-04, LAN: 427 passed, 12 failed)